The present invention relates to a novel thermoplastic elastomer composition superior in fluidity and external appearance of molded articles. More particularly, it is concerned with a thermoplastic elastomer composition obtained by partially cross-linking a composition of (A) an extremely low density ethylene copolymer prepared by copolymerizing ethylene and an .alpha.-olefin in the presence of a specific catalyst, (B) a propylene polymer, (C) an ethylene/.alpha.-olefin copolymer rubber and (D) an ethylene/unsaturated monocarboxylic acid ester copolymer, the said thermoplastic elastomer composition being superior in fluidity, external appearance of molded articles and resistance to heat and oil, and being small in permanent set.
As thermoplastic polyolefin elastomers there are known compositions comprising crystalline polyolefins such as polyethylene and polypropylene as hard segments and amorphous copolymer rubbers such as ethylene/propylene copolymer rubber (EPR) and ethylene/propylene/non-conjugated diene copolymer rubber (EPDM) as soft segments, as well as compositions obtained by partially crosslinking the above compositions. It is also known to prepare hard and soft segments according to a multi-stage polymerization process. And by changing the proportions of those segments there are obtained various grades of products ranging from one superior in flexibility up to one having rigidity.
Products of the flexible grade are attracting great attention because they can be applied as rubbery materials widely to such uses as automobile parts, hoses, electric wire coating and packing. In preparing such flexible grade of products it is necessary to increase the proportion of a soft segment (e.g. EPR or EPDM) and decrease that of a hard segment (e.g. polyethylene or polypropylene) in order to impart rubbery flexibility thereto.
However, such soft segments as EPR and EPDM are poor in tensile strength and inferior in resistance to heat and oil and also inferior in fluidity. Consequently, flexible, thermoplastic elastomer compositions containing large amounts of such soft segments also have the above-mentioned drawback and cannot be applied to a wide variety of uses. Increasing the hard segment proportion to remedy these problems will result in loss of flexibility, deterioration of physical properties such as permanent set and consequent impairment of the function as a flexible, thermoplastic elastomer.
Moreover, in preparing a product of the flexible grade, it is necessary to carry out polymerizations separately for hard and soft segments, thus resulting in that not only the polymerization apparatus becomes very complicated in structure but also it is very difficult to control the properties and proportion of each segment in each polymerization stage and a defective product sometimes occurs at the time of changeover from one to another grade. Further, the recovery of the resulting polymer is also very difficult because a large amount of a rubbery component is contained therein.
Thus, a lot of problems remain to be solved in order to obtain a flexible, thermoplastic elastomer of good quality.